(1) Technical Field
The present disclosure is related to optical writing devices and image forming devices. In particular, the present disclosure is related to a technology of suppressing a fluctuation in a light amount emitted by a light-emitting element that otherwise occurs when a voltage droop occurs in a thin film transistor (TFT) that supplies the light-emitting element with a drive current.
(2) Related Art
Recently, in order to achieve a reduction in image forming device size and cost, development is in progress of organic light-emitting diode (OLED) print heads (PHs), which are optical writing devices including OLEDs arranged in line(s) as light sources. OLED-PHs contribute to reduction in image forming device size and cost because in an OLED-PH, light-emitting elements (OLEDs) and drive circuits (TFTs) are disposed on the same substrate.
In connection with this, TFTs containing low-temperature polycrystalline silicon (LTPS) (such TFTs referred to in the following as LTPS-TFTs) are known. In an LTPS-TFT, when a voltage greater than a threshold voltage Vth is continuously applied between the source and gate electrodes (the voltage between the source and gate electrodes of an LTPS-TFT is referred to in the following as a voltage Vgs), a so-called voltage droop occurs. That is, in an LTPS-TFT, the longer a voltage greater than the voltage Vth continues to be applied between the source and gate electrodes, the smaller the current between the source and drain electrodes becomes.
Thus, when LTPS-TFTs are used as TFTs in an OLED-PH, a light amount that an OLED emits decreases as the amount of time increases for which the OLED is kept in on state, as illustrated in portion (a) of FIG. 12. This produces a sub scanning direction density unevenness. For example, when a solid image is being formed, the density of the solid image decreases as image forming progresses in the sub scanning direction, as illustrated in portion (b) of FIG. 12.
As an example of a countermeasure against this issue, Japanese Patent Application Publication No. 2002-144634 (referred to as “Patent Literature 1” in the following) discloses a technology of overcoming such density unevenness by measuring OLED light amounts by providing light amount sensors, one for each OLED, and performing feedback control based on the OLED light amounts.
However, the conventional technology disclosed in Patent Literature 1 requires providing a plurality of sensors, one for each OLED, and thus inevitably leads to an increase in OLED-PH size and cost.